1. Field of the Invention
The invention relates to a room temperature crosslinking, moisture cured, one component (RTV 1C) silicone rubber formulation with hydrophobic silica, featuring extremely low water absorption, a high level of whiteness, fine-tunable, storage-stable rheological properties and a high reinforcing action in the silicone rubber vulcanizate.
2. Discussion of the Background
The use of silicas, especially hydrophobicized precipitated silicas, in silicone rubber formulations is known and is described in, for example, DE 27 29 244 and DE 26 28 975.
DE 26 28 975 and DE 27 29 244 describe preparing hydrophobic silicas by reacting a hydrophilic precipitated silica featuring low water absorbency with silicone oil or dimethyldichlorosilane, respectively. In the process according to DE 26 28 975, the reaction is carried out with the hydrophobicizer (silicone oil) being added to the dry precipitated silica. In the process according to DE 27 29 244, the hydrophobicizer (dimethyldichlorosilane) is introduced directly into the precipitated silica suspension. In both cases, the hydrophobicizing step is followed by heat treatment at elevated temperatures, specifically between 200 and 400° C.
A disadvantage of this process is that the precipitated silica thus hydrophobicized becomes discolored at the required process temperatures. The discoloration of the silica is particularly inconvenient when it is added to silicone formulations; that is, when these hydrophobic precipitated silicas are added to silicone rubber formulations or to defoamer mixtures based on silicone oil.
As a measure of the discoloration it is possible to use the value known as reflectance. In measuring the reflectance, the diffuse reflection power of a sample is investigated. The higher the diffuse reflection power of the sample, the higher its reflectance and thus the higher the whiteness of the sample.
Precipitated silicas generally have a reflectance of not more than 97%. One object of the present invention is therefore to prepare an RTV 1C silicone rubber formulation comprising hydrophobic silica which has a high reflectance.
Discoloration occurs in particular with the hydrophobicizing of silicas when the silicas are strongly hydrophobicized: that is, have a high methanol wettability and high carbon loadings. It is precisely these properties, however, which are desired in RTV 1C silicone rubber formulations.
In order to fully develop their mechanical properties RTV 1C silicone rubber formulations need active reinforcing fillers. At present, only highly disperse pyrogenic silicas are used. A reinforcing filler must effect good mechanical properties in the vulcanizate, and the rheological properties set must remain stable even after prolonged storage.
RTV 1C silicone rubber formulations are processed at room temperature. In the case of silicone rubber formulations which cure by means of atmospheric humidity, too high a water content in the filler results in unwanted curing in the course of storage and in a low-grade surface of the vulcanizate. Accordingly, the water absorption characteristics, i.e., the amount of water adsorbed at different relative atmospheric humidities, constitute a measure of the processability of the filler.
A disadvantage of the known silica hydrophobicizing processes is that only a limited amount of hydrophobicizer can be attached covalently to the silica. Particularly in RTV 1C silicone rubber formulations, however, high carbon loadings are desired, since they permit establishment of and decisive improvements in the processing properties and/or the dispersing of the silica and also in the rheological properties, such as the thickening, i.e., the desired yield point and viscosity, of the compounds.
As a measure of the thickening, it is possible to utilize the DBP number. The DBP number indicates the absorption capacity of a silica for DBP. The measurement technique shows the amount of dibutyl phthalate, in g, of a sample of 100 g, at which a sharp increase in force in the compounder is observed.
The thickening action of the known hydrophobic precipitated silicas is probably attributable to their low carbon contents of less than 3.1% and/or to inhomogeneous hydrophobicizing. This lowers the maximum possible fraction of silica as filler in silicone rubber formulations. DE 26 28 975 lists data on the testing of hydrophobic precipitated silicas in silicone rubber formulations, in which the hydrophobic precipitated silica is used in increasing weight fractions. All the tables indicate that the mechanical properties of the silicone rubber vulcanizates are improved as the filler content goes up.
It would therefore be desirable to prepare RTV 1C silicone rubber formulations which include high fractions of hydrophobic silicas, for improving the mechanical properties, but which at the same time are still flowable and also feature good storage stability.